make "rebuild Project" (C-c C-b r) work again

Explanation:
- the "defsetf" in ensime-def-connection-var evaluated the "store"
parameter at the wrong time
- because of that, "buffer-file-name" in
ensime-builder-track-changed-files was being evaluated inside
"ensime-with-connection-buffer" (thus evalutating to nil)
- because of that the buffer file name wasn't being saved